Why do we eat grains? Thermodynamics!

Around 12,000 years ago, a group of Neolithic humans living in what is now the Middle East conducted a remarkable experiment.

For generations their people had collected the seeds of tall grasses, which they cooked and ate to supplement their diet of foraged fruits, vegetables, and leaves, and hunted game, fish, and seafood.

Some years, due to drought or animal migration, food was harder to find. They adapted by moving to new areas or reorganizing their hunting and foraging parties. But venturing into new ground meant encroaching on the territory of others, and when a rival tribe was unwilling to cooperate the result was often war².

So these early humans tried a new way of dealing with unreliable food supplies. They gathered seeds from the tall grasses and planted them within their own territory. They watched over them, weeded out other plants, and did their best to fend off birds and other pests. The plants survived long enough to produce more seeds. The experiment was a success.

So began the most significant cultural and social upheaval in human history.

The Neolithic Revolution

Nutritional anthropologists call this the Neolithic Revolution. Why exactly it occurred is still a matter for debate; the life of a Neolithic farmer was not, on the face of it, a radical improvement on the life of a Neolithic hunter-gatherer. Early agriculture must have been back-breaking and unreliable to a degree scarcely imaginable to modern minds. Tools would have been primitive. Much of the labor — planting, weeding, watering, harvesting, etc. — would have been performed by hand. Crops would have failed often for reasons the neolithic cultivators could not possibly have understood.

Moreover, the food itself was less nutritious. Early agriculturists were less healthy and had a lower life expectancy than their hunter-gatherer peers despite having access to more food energy³.

But the significant negatives of early farming were outweighed by the positives: a dense source of calories produced in small area, and a foodstuff that could be stored for long periods.

Before the Neolithic Revolution, the population density of a typical hunter-gatherer group was less than 1 person per square mile. Making the reasonable assumption that the human population was limited primarily by the amount of food available⁴, we can discern that the number of available calories (kcals) per square mile of land was something under 2500. A few centuries later, the population density of very early agriculturists was on the order of 20 per square mile, implying the number of calories generated per square mile of land was at least 50,000. As a first-order approximation, the earliest attempts at agriculture were 20 times more efficient than hunting and gathering at generating calories for a given unit of territory, even after accounting for bad harvests and failed crops.

Scholars continue to argue over whether an increase in population made the shift towards agriculture necessary, or a shift towards agriculture caused an increase in population that made a return to a hunter-gatherer society impossible⁵. In either case, once the neolithic revolution had begun there was no turning back.

Everything changes

Even in the short term, the changes were immediate. With more food available within their own territory, groups grew larger and moved around much less. Women were able to have more children and have them earlier². Groups became increasingly territorial and the concept of land ownership became more firmly entrenched². A hunter-gatherer lifestyle could no longer produce enough calories per person to support the swelling population.

In the long term, the shift towards agriculture changed the course of human development to an extent difficult to exaggerate. Crop cultivation led to crop domestication and the eventual domestication of the wild sheep and goats that were once only hunted. As the centuries passed, a wider variety of grains and legumes, and eventually tubers, fruits and vegetables, were cultivated and domesticated in other parts of world (notably the Yangzi river basin in China² and later in Africa and Mesoamerica). The capability of producing food surplus to immediate dietary need allowed for trade, tributes and taxes. For the first time in human history, large numbers of able-bodied adults could devote themselves to pursuits other than finding food, allowing professions like craftsmen, priests and bureaucrats to develop⁶. It allowed villages and towns to grow to sizes otherwise impossible. And more than anything, it allowed the human population to expand at an incredible rate.

Grains, land and calories

But as human command of agriculture grew increasingly accomplished, why did some crops fuel the growth of empires while others remained relatively unimportant? Why did grains like wheat, corn and rice become a daily staple for hundreds of millions of people?

The answer has more to do with thermodynamics than anthropology. Grains produce more calories per acre⁷ than almost all other foodstuffs. (The biggest exception to this rule is the common potato, which helps to explain why it’s the fourth most commonly consumed food on the planet behind rice, wheat and corn).

Take the USDA National Agricultural Statisics Service  2012 figures for average yield per acre for some of the most frequently consumed crops in the USA. Multiply them by the National Agricultural Library figures for average calorie (kcal) content per raw 100 grams. You end up with a table a little like this:

Crop	Yield per acre (1000 lbs)	Calories per 100 grams (kcal)	Calories produced per acre (millions)
Potatoes	41.2	77	12.9
Rice	7.7	365	12.3
Corn	6.9	365	11.4
Onions	48.7	40	8.8
Apples	27.6	52	6.6
Oranges	30.1	47	6.4
Carrots	30.9	41	5.7
Soybeans	2.4	446	4.8
Wheat	2.8	340	4.3
Grapes	15.2	57	4.0
Peaches	17.3	39	3.1
Broccoli	16.2	34	2.5
Tomatoes	29.1	18	2.4
Lentils	1.2	353	1.9

This is, of course, an extremely rudimentary measure of how many people a crop can feed. It doesn’t take into account how much water or fertilizer different crops need. It doesn’t control for the different levels of labor and expense each crop requires. But it does give a general indication of why so much of the world’s diet is based on grain.

Imagine you have a group of 100 people to feed for a year and that they need each need 2000 calories per day to survive. You calculate you need about 73 million calories of food energy⁸.

If your food crop is corn, you need about five and half acres of land and storage for about 19 tons of food. If it’s rice, you need six acres and 23 tons of storage. With wheat, you need 17 acres and 24 tons of storage.

If your food crop is potatoes, you only need about five and half acres, but you somehow have to find a way to store 117 tons of food. (Food that can’t easily be dried and stored for long periods, like the cereal crops). Onions? Eight acres but 200 tons. Lentils? 38 acres and 23 tons. Broccoli? 29 acres and 230 tons.

Different needs

The advantages of cereal crops are manifest from this perspective. But it’s a perspective that is increasingly less relevant to people living in affluent developed countries where food shortages are rare and population growth is slowing⁹. For millenia, food output has been one of the primary limits on population growth¹⁰. In the developed world, that’s simply no longer the case.

But diets haven’t adapted. In fact, the average American ate 60lb more of grain products per year in the year 2000 than in the 1970s¹¹. True, Americans also ate about 120lb more fruit and vegetables¹¹ by the same metric, but as we’ve established grains are much more calorie-dense than most other crops. In other words, that 60lb of grain products represents a much greater portion of a person’s caloric budget than the 120lb of produce.

It’s not surprising that grains still make up such a large portion of the calories people in the developed world consume. Our entire food culture has been underpinned by the need to eat grains for thousands of years. Everything about food — our recipes, our tastes, our idea of what’s appropriate for breakfast, lunch and dinner, our idea of an appropriate number of meals per day, our idea of an appropriate quantity of food to eat — has developed and evolved around a grain-based diet.

But there is nothing intrinsic to breakfast that demands cereal in a bowl of milk. There is nothing intrinsic to lunch that demands bread. There is nothing intrinsic to dinner that demands rice. There is nothing intrinsic to eating that demands a meal’s worth of calories should fit on a plate of a certain size or take a certain amount of time to consume.

Food production has been optimized throughout the entirety of human existence. But it has been optimized to solve a problem most people reading this article no longer experience. Society no longer needs more food; it needs healthier food.

Eating fewer grain products is not synonymous with popular diets like low-carb or gluten-free. It’s possible to eat fewer grain products without altering the ratio of macro-nutrients one consumes or worrying about the occasional slice of bread. It needn’t be borne by commitment to a food movement like paleo.

It can stem from the simple proposition that fruits and vegetables are generally healthier to eat than grains. That’s an argument I’ll cover more deeply in later posts.

I don’t think grain products are evil and I accept that humans have eaten grain-based diets for thousands of years. But I believe it’s important to understand why that has been the case, and to recognize the unique change of circumstances that has occurred in the developed world in the last century.

Around 12,000 years ago, a group of Neolithic humans conducted an experiment that forever changed the course of human development. They left behind the way they had eaten for countless generations and moved to a diet that helped them face new problems and evolve in new ways. I believe it’s possible for this generation to do the same.

Interested in republishing this post elsewhere? Email me!

1. Photo credit: José-Manuel Benito Álvarez [license] [↩]
2. On the Nature of Transitions: the Middle to Upper Palaeolithic and the Neolithic Revolution [↩] [↩] [↩] [↩]
3. Ancient Health: Skeletal Indicators of Agricultural and Economic Intensification [↩]
4. The human motivational complex: evolutionary theory and the causes of hunter-gatherer fighting [↩]
5. From foraging to farming: Explaining the Neolithic revolution [↩]
6. The Agricultural Revolution in Prehistory : Why did Foragers become Farmers? [↩]
7. I’ll use acres rather than square miles for ease of conversion — the USDA publishes yield statistics in bushels per acre [↩]
8. 2000 * 100 * 365 = 73,000,000 [↩]
9. U.S. on Pace for Slowest Decade of Population Growth Since 1930s [↩]
10. Population, Food, and Knowledge [↩]
11. USDA Factbook Chapter 2 [↩] [↩]